A Liquid Crystal Display (LCD) generally consists of a Liquid Crystal Module (LCM) and a Back Light Module. As liquid crystals do not generate light by themselves, the back light module provides the light source required by the LCM to accomplish display functions. The back light module in the LCD generally consists of a Cold Cathode Fluorescent Lamp (CCFL), a metal lampshade, a reflector, a light guide panel (LGP), a plurality of optical films (such as diffusers), a plastics frame and a metal frame. The LGP usually is made from Polymethylmethacrylate (PMMA) by injection forming to form a square plate with smooth surfaces. The square plate has a thicker side and a thinner other side with the thickness therebetween tapering gradually. The bottom side of the LGP is printed with material which has high reflection and no light absorption properties by silk-screening to form diffusion spots. The thicker side of the LGP is corresponded to a light source. Light emitted from the light source transmits to the thinner side by reflection. When the light projects on the diffusion spots, reflected light diffuses in all angles. The condition of reflection is broken. Light emits from the front side of the LGP. By forming the diffusion spots in dots of different densities and sizes, the LGP may illuminate evenly. As the function of the LGP is to scatter light in different directions to increase luminosity and achieve uniform brightness of the LCM, the quality of the LGP has a great impact on the back light module. With improved manufacturing processes and material, the yield of the LGP has reached a desired level. Coupled with suitable assembly, the back light module can provide a uniform light source for the LCM.
On the back light module, the metal lampshade is located on one side of the plastics frame. The light source is housed in the metal lampshade. The reflector, LGP and optical films are slightly smaller than the plastics frame. The reflector is firstly mounted on the bottom of the plastics frame. Then the thicker side of the LGP is wedged in the metal lampshade. The optical films are stacked on the LGP in a laminated fashion. The plastics frame prevents the LGP from moving in the directions of the X-axis and Y-axis. In addition, the plastic frame has latch blades extended inwards corresponding to the corners of the metal lampshade to latch the edges of the LGP to prohibit the LGP from moving in the direction of the Z-axis. Thereby the LGP is confined in the plastics frame. Finally, the metal frame encases the plastics frame from the outside to complete the back light module.
The assembling structure of the back light module set forth above has some critical problems: first, the distance between the thicker side of the LGP and the light source is critical. The LGP must maintain a definite distance with the light source to allow light emitted by the light source to enter the LGP; second, the thicker side of the LGP must be coupled with the metal lampshade properly. That is the thicker side of the LGP must be completely wedged into the metal lampshade to prevent the light emitted from the light source from scattering out through the juncture. The aforesaid concerns basically can be resolved by precisely controlling the dimensional relationships such as the dimensional relationship of the LGP and the plastics frame, metal lampshade and wedging position of the LGP, and the coupling relationship between the latch blades and the LGP. However, the actual situation does not always happen as desired. For instance, material properties of the LGP, use conditions of the LCD or environmental conditions might change the dimensional relationships. In order to prevent damage from occurring to the back light module due to use conditions or environmental conditions, vendors of back light modules usually have to perform reliability tests in various use and environmental conditions such as high temperature, high humidity, or the like to ensure that the back light module can overcome the factors being encountered. The LGP made from PMMA, for example, will absorb water and expand and deform. The expansion rate of PMMA is about 0.3%. To resolve the expansion problem of the LGP after absorbing water, a gap is generally created between the LGP and the plastics frame to give the LGP a desired expansion space in the high humidity environment and prevent the LGP from warping after absorbing water and expanding. The gap design creates other problems. For instance, if the gap is too large, the plastics frame can better accommodate the environmental conditions but is less capable to accommodate the use conditions. On the other hand, if the gap is too small, the plastics frame can better accommodate the use conditions but is less capable to accommodate the environmental conditions. For instance, when the LCD is used on notebook computers, which are often carried outdoors and for which the display device must be lifted for use and closed when not in use, the gap should be as small as possible to avoid the thicker side of the LGP of the back light module from separating from the metal lampshade. However, if the gap is smaller, when the notebook computers are used in areas of high humidity, the deformation caused by expansion of the LGP is smaller, and the LGP tends to warp. This severely impacts the display effect.